If you have a wallet.dat from the original client. How easy is it to transfer this wallet to other clients?
They generally are not compatible. The Bitcoin.org client is generally backwards compatible (a current release of the Bitcoin.org client can use an older version of the wallet.dat) but a Bitcoin.org wallet.dat can't be used by MultiBit, for instance.
If you want to transfer keys, you'll need to export and import.
For exporting you might use the BitcoinTools and do dumpwallet
For linux, ./dbdump.py --wallet
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https://github.com/gavinandresen/bitcointoolsImport instructions will vary based on which client you are referring to.
At some point the wallet.dat in the Bitcoin.org clinet will be going away, replaced with some other type of file.